1. Field of the Invention
The present invention generally relates to a cooling system of an engine for personal watercraft. More particularly, the present invention relates to a cooling system which is capable of inhibiting excess cooling of a cylinder block of an engine.
2. Description of the Related Art
In recent years, water-jet propulsion personal watercraft have been widely used in leisure, sport, rescue activities, and the like. A typical personal watercraft comprises a body including a hull and a deck covering the hull from above, and is equipped with an engine in a space inside the watercraft that is defined by the hull and the deck. The engine is configured to drive a water jet pump configured to propel the watercraft, which pressurizes and accelerates water sucked through a water passage from a water intake generally provided on a hull bottom surface and ejects it rearward from an outlet port. As a result, the personal watercraft is propelled.
In such a personal watercraft, typically, water, for example, sea or lake water that has been pressurized by the water jet pump is partially drawn up from an inside of the water jet pump through a water-drawing hole formed in a pump casing of the water jet pump, for use as cooling water to cool various engine components. The cooling water flows through a cylinder head, a cylinder block, and auxiliary devices such as an exhaust pipe (e.g., exhaust manifold) while cooling these components. Such a cooling system is referred to as an open-looped cooling system (direct cooling system). In the personal watercraft disclosed in Japanese Patent No. 3084781, water is taken in from outside the watercraft for use as cooling water and is supplied to a passage formed in a wall portion of an exhaust manifold, and some of the cooling water flows through a passage formed within a wall portion of a cylinder block and then through a passage formed within a wall portion of a cylinder head while cooling the engine components. After that, the cooling water is discharged outside the watercraft.
Since the exhaust pipe and the cylinder head tend to heat up to relatively high temperatures, they must be sufficiently cooled. On the other hand, if the cylinder block is cooled excessively, then a friction loss may increase between a piston that slides within the cylinder block and an inner wall of the cylinder block, and further, fuel existing within a combustion chamber may not be sufficiently vaporized, and the unvaporized fuel may flow into lubricating oil, leading to dilution of the oil.
In the personal watercraft which employs the above mentioned open-looped cooling system, the water used as cooling water, which is generally sea or lake water, typically has a relatively low temperature, and thus the cylinder block tends to be cooled excessively, if the engine runs at a low engine speed during, for example, an idling state in which heat generation amount is small.
In the cooling system disclosed in the above patent Japanese Patent No. 3084781, a thermostat is equipped to control a flow rate of the cooling water supplied to the engine components to inhibit excess cooling of the engine. In this cooling system, however, since the thermostat is positioned downstream of a point at which the cooling water that has cooled the cylinder head and the cooling water that has cooled the cylinder block gather, the flow rate of the cooling water to be supplied to the cylinder head may be reduced if the thermostat operates to inhibit excess cooling of the cylinder block. As a result, the cylinder head may be undesirably cooled insufficiently.